There is known a liquid crystal optical element which uses the birefringence of liquid crystal molecules to change the refractive index distribution in response to voltage application. There is also a stereoscopic image display device including such a liquid crystal optical element in combination with an image display unit.
The stereoscopic image display device is switched between two states by changing the refractive index distribution of the liquid crystal optical element. In one state, the image displayed in the image display unit is directly projected on the observer's eye. In the other state, the image displayed in the image display unit is projected as a plurality of parallax images on the observer's eye. This realizes the operation of displaying a high-definition two-dimensional image and the operation of displaying a three-dimensional image for naked-eye stereoscopic viewing by means of a plurality of parallax images. In the liquid crystal optical element used in the stereoscopic image display device, it is desired to realize good optical characteristics.